<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 300px;
            height: 50px;
            border: 1px solid #f00;
        }

        .bar {
            width: 5%;
            height: 50px;
            background-color: #f00;
            transition: all .3s;
        }

        .box:hover .bar {
            width: 100%;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="bar"></div>
    </div>
</body>

</html>